High-energy density power storage devices typified by a lithium ion secondary battery, a lithium ion capacitor, an electric double layer capacitor, and the like, are structured, for example, such that a power storage element configured by stacking a sheet-like electrode formed by coating a sheet-like current collector foil (such as aluminum foil or copper foil) with an active material (such as activated carbon, a lithium composite oxide, or carbon), with a sheet-like separator interposed therebetween for preventing short circuit due to contact between electrodes; and an electrolyte solution are housed in an outer packaging composed of an aluminum can, an aluminum laminate film or the like.
(1) Prior Art Using Separator of Polyethylene Microporous Membrane
As the power storage device described above, a lithium ion secondary battery has been proposed which uses a separator of a polyethylene microporous membrane (see Non-Patent Document 1).
It is common to use, as a separator in a lithium ion secondary battery, a polyethylene microporous membrane as described in Non-Patent Document 1. This is due to the fact that when the lithium ion secondary battery generates abnormal heat, the polyethylene is melted to block the micropores, thereby suppressing or preventing lithium ion permeation to shut down the current, so that a safety mechanism can be incorporated in the lithium ion secondary battery.
As the polyethylene microporous membrane described in Non-Patent Document 1, a monoaxially-oriented or biaxially-oriented film is typically used for improvement in strength; however, the stretching treatment of the film increases the crystallinity of the polyethylene, and also increases the shutdown temperature to around the thermal runaway temperature of the battery, leading to a problem with reliability.
In addition, there is a problem that strain is accumulated in the film by the stretching treatment, and when the film is exposed to high temperatures, the film is significantly shrunk by the residual stress.
Furthermore, an increase in energy density for lithium ion secondary batteries has been desired in recent years, and in the case of such higher-energy density lithium ion secondary batteries, for the reason mentioned above, it is difficult to ensure adequate safety with a polyethylene microporous membrane, and above all, it goes without saying that film shrinkage due to residual stress is fatal.
(2) Prior Art Using Separator of Composite Material Including Inorganic Powder
(2-1) As a separator that is able to suppress such shrinkage as caused in the case of the polyethylene microporous membrane described above, a ceramic microparticle composite separator has been proposed which has ceramic microparticles (0.01 μm or 0.3 μm in grain size) and a binder resin combined with predetermined PVC (pigment volume concentration) (see Non-Patent Document 2).
In addition, patent documents also disclose the following separators as separators that are able to suppress shrinkage of resin layers:
(2-2) a microporous separator mainly containing a mixture of olefin plastic and hydrous silica (see Patent Document 1);
(2-3) a separator structured such that a resin layer is provided on at least one principal surface of a base material layer, where the resin layer includes an inorganic substance in a range of 1 nm to 10 μm in particle size (see Patent Document 2); and
(2-4) a separator containing inorganic microparticles in which the number of particles of 0.3 μm or less in particle size and the number of particles of 1 μm or more in particle size are each 10% or more to the total number of inorganic microparticles (see Patent Document 3).
In the case of a separator of a composite material composed of an inorganic powder and an organic binder, such as the separators described in Non-Patent Document 2, and Patent Documents 1 to 3, it becomes possible to reduce the shrinkage problem as described above.
One of important functions of separators is the function of permeating ions (lithium ions in the case of a lithium ion secondary battery) while preventing short circuit between electrodes, and the separators in Non-Patent Document 2 and Patent Documents 1 to 3, which are prepared by binding, with the use of an organic binder, gaps filled with an inorganic powder, need adjustments to porosity and air permeability in order to secure the function of permeating lithium ions, and to that end, need proper adjustments to the filling property of the inorganic powder.
When an inorganic powder with a particle size distribution is packed as the foregoing inorganic powder, for example, smaller particles penetrate into gaps among larger particles as the particle size distribution is increased in breadth, and the porosity is thus decreased to make the powder densely packed.
However, Non-Patent Document 2 and Patent Documents 1 and 2 fail to describe the particle size distribution of the inorganic powder, and actually fail to present any specific measures for adjusting the porosity and air permeability of the separator.
On the other hand, Patent Document 3 describes the particle size distribution, but there is a concern that the inorganic powder is densely packed to make it impossible to ensure the lithium ion permeability required for the separator, because the configuration in Patent Document 3 provides the inorganic microparticles with a wide particle size distribution.